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A realistic model for estimating software project time would account for division of staff time among several competing projects and would model individual productivity. A good model would also adequately address software rework due to errors and changes in requirements identified before or after installation. Specifically it should consider that rework could require effort in any previous phase, and not just the preceding one. Correction of an error or implementation of a change could involve only part of the software. While that part is undergoing rework the rest could continue to later phases of development. These characteristics find a close analogy in the job shop model which is commonly used in manufacturing. In the adaptation of this model to software, jobs (software items needing work) are handled by machines (people). The model is represented by a stochastic network which routes jobs from multiple projects through the phases of a modified waterfall life cycle. It handles rerouting due to errors, post installation work and individual productivity levels. When calibrated for a sample firm, the model performed rather well in predicting project effort and installation dates.
Tools and techniques, management of computing and information systems, software management, predicting project effort

C. A. Leonhard and J. S. Davis, "Job-Shop Development Model: A Case Study," in IEEE Software, vol. 12, no. , pp. 86-92, 1995.
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